Strip the paint from the stairs. Apply paint stripper or heat the paint with a heat gun, and scrape the paint away with a putty knife when it separates from the wood. If this is a new staircase, skip this step.
Sand away any paint spots that you couldn't remove with the stripper or heat gun and putty knife, using 220-grit sandpaper.
Clean the wood by pouring mineral spirits onto a clean rag and wiping it over the steps. Allow the steps to dry.
Apply the stain to the stairs, beginning at the top of the staircase and working down. Use smooth, even strokes when applying the stain, fully covering an area about 6 inches long with the stain before moving on to the next section. Run the paintbrush over the edges of the stairs and any places where there is excess stain. Allow the stain to dry for the time specified on the packaging.
Apply a second coat of stain to the stairs to darken them further, if desired.